Spool valves are generally well known in the art of pneumatic controls, and particularly railway air brake systems where spool valves are commonly utilized as release valve assemblies in freight car air brake control systems. These spool valves are normally characterized by a cylindrical valve stem, having one or more annular channels in its periphery, which is slidably secured within a cylindrical opening in a housing body. These spool valves are pneumatically controlled to lift or lower the valve stem to change the alignment of the annular channel, or channels, with different air passageways within the walls of the cylindrical opening, as necessary, to change the air flow between one pair of air passageways to another pair of air passageways.
Commonly, O-ring seals are provided at each end of each annular channel and between adjacent annular channels to assure air tight seals between the valve stem and the cylindrical opening and particularly between adjacent air passageways. The pneumatic valve control is normally achieved by providing a diaphragm at one end of the valve stem which separates a pair of diaphragm chambers, so that pressurized air admitted to either one of the two diaphragm chambers will cause the diaphragm to deflect one way or the other to lift or lower the valve stem as necessary to change the alignment of the annular channel or channels with respect to the various air passageways in the walls of the cylindrical opening.
In the case of release valves assemblies in a freight car air brake control system, the diaphragm is normally retained in place at one end of the valve stem by fitting an axial aperture in the diaphragm over a small threaded axial extension at the end of the valve stem, and then compressing the diaphragm onto a radial flange at the base of the threaded extension with a nut and washer combination tightened onto the threaded extension. In using a threaded nut to compress the diaphragm onto the radial flange, an exceptionally tight seal can be achieved between the diaphragm and the valve stem thereby assuring that compressed air cannot leak therearound.
Additionally, in the case of release valves assemblies in these freight car air brake control systems, it is common to provide an axial bore through the full length of the valve-stem, which, accordingly, must also pass through the threaded extension at the end of the valve stem. The drilling of the axial bore is naturally complicated by the exceptional length thereof, the small nature of the threaded extension and the minimum volume of material contained in the threaded extension. Accordingly, the normal practice is to drill a bore of two different diameters, a large diameter through the spool portion of the valve stem and a small diameter through the extension portion of the valve stem.
These same limiting characteristics can also lead to problems in assembling the spool valve and problems even later in service, resulting in fractured valve stem extensions or damaged threads, which thereby prevent the ability to achieve a tight seal between the diaphragm and the valve stem.